Method of chromium plating



Patented Oct. 2, 1934 UNITED STATES PATENT OFFICE METHOD OF CHROMIUM PLATING many No Drawing. Application June 29, 1933, Serial No. 678,258. In Germany October 16, 1929 5 Claims.

The invention relates to methods of chromium plating.

In practice it is often necessary to deposit on metals dark or black layers which show a good 5 absorption or radiation of heat. For instance, it

is well known to darken the surface of the electrodes of electron tubes. To give an example nickel may be plated with nickel oxide, which has a much greater tendency of emitting rays. The

re use of nickel oxide, however, is not suitable in many cases because its melting as well as its decomposition takes place at relatively low temperatures. Besides the nickel oxide has a remarkable inconstancy to the bombardment by ions or by electrons.

It is also possible to produce pale-dark coloured coatings of metals having a higher melting point and a greater constancy. Chromium, for instance, can be electrolytically deposited in such a form by diminishing the anodic current density and thereby varying the distribution of the current lines within the bath.

Grey coloured chromium layers can be obtained by increasing the cathodic current density very much with respect to the values usual in practical chromium plating, that means about 50 amperes per square meter of cathode surface. But the grey colour of the chromium coatings only happens when passing the current through the bath for a short time and disappears afterwards when depositing long enough so as to make/an homogeneous layer.

Both the grey and the dark chromiumcoating show insufficient qualities for the emission of rays.

According to this invention a layer of quite black colour and of excellent qualities for the said purpose is deposited by the use of a cathodic current density from ten to twenty times higher than the usual one, that is lying between about 10,000

and 20,000 amperes per square meter. Chromic acid containing baths may be employed if desired.

The exact composition of this black deposit is not exactly known even now. It is supposed that it consists of chromium oxide or of metallic chromium in a very fine distribution or perhaps of both these components. The qualities of this electrolytically produced coating are a very great constancy and capacity of resistance to temperatures up to 2000 C. It is of importance that in the said layer no vapours or gases are present or can be developed during the normal use of a so treated article. Such gases could give disturbing effects and eventually destroy other parts of the apparatus.

Careful investigations have been undertaken to find out the best method of producing the black deposits above described. The metal the surface of which is to be darkened can be treated as a cathode in a chromium bath, for instance, consisting of a solution of chromic acid (H2CrO4) 0 and very small percentage of sulphuric acid (S04), using a current density of about 15,000 amperes per square meter. Experiments have shown that the utmost upper limit of the sulphuric acid (S04) content lies at 0,07 per cent. In many cases precious black chromium containing layers are obtained. In other cases, especially if a good protection of corrosion is desired simultaneously,

it is preferable to treat the article at first in a normal chromic acid bath until a white or bright chromium coating appears. On this first chromium deposit a second layer of black colour is separated out of a chromium bath free of sulphuric acid (S04) by the use of a current density of 10,000 up to 20,000 amperes per square meter. It is remarkable that articles which have been first chromium plated in the usual manner give a very good throwing power in the second bath suitable for black chromium plating. The first layer on the article to be treated may also consist of electrolytically deposited nickel upon which the dark coloured chromium containing layer is formed. In special cases several intermediate layers can be used, for instance, a first nickel layer and a second white chromium layer which serves as a base for the dark deposit.

Bright chromium deposits of black colour are obtained out of normal chromic acid baths suitable for chromium plating by removing the sulphuric acid (S04) therefrom as completely as possible. This can be executed in any well known manner, for instance, by the chemical precipitation with barium compounds which are added to the chromium plating bath.

If organic acids are introduced into a standard chromic acid bath after separating therefrom the sulphuric acid grey-black deposits can be produced by electrolysis, such deposits being very proper to serve as a source for the emission of rays or of electrons. It was found that carbonic acids such as acetic acid, formic acid, propionic acid and oxalic acid can be used as additions to standard chromium plating baths from which sulphuric acid is removed.

The new method is applicable for blackening metallic articles in order to change their power of emission as for instance at thermic apparatus, screens, electrodes or other metallic parts of vacuum tubes, especially anodes of discharging tubes of high effect. no

Practical examples 1. Tungsten electrodes of electronic tubes are plated with white chromium layers in the usual manner. After rinsing the so prepared tungsten electrodes for a short time, they are treated in a solution containing 35 per cent of chromic acid and 0,05 per cent of sulphuric acid (S04) by means of an electric current. The current density amounts to about 10,000 amperes per square meter and the voltage 10 volts. The temperature of the bath is maintained at below 20 C., if necessary by cooling. After about ten minutes the tungsten article connected as cathode carries a black chromium coating.

2. From a standard chromium plating bath which contains chromic acid 350 grams per liter and a small quantity of sulphuric acid ($04), the sulphuric acid (S04) is at first separated in any well known manner. Thereafter from 0,5 to 0,8 cubic centimeters of concentrated acetic acid per liter are added to the bath. The electrolysis was performed with a cathodic current density of 10,000 to 20,000 amperes per square meter, a voltage of about 9 volts and a temperature of bath below 28 C. The anode was made of lead and the cathode was formed by a tungsten article. The result was a pale-black cathodic layer of chromium showing excellent qualities of heat or electron emission.

What we claim is:

1. Method of producing black chromium coatings by electrolysis in a chromium electrolytic bath, using a current density of from 10 to 20 times higher values than heretofore proposed, that is about 10,000 to 20,000 amperes per square meter.

2. Method according to claim 1, in which a chromic acid solution substantially free of sulphuric acid is electrolyzed.

3. Method according to claim 1, in which a chromic acid solution substantially free of sulphuric acid, but containing an addition of organic carbonic acid is electrolyzed.

4. Method of producing black chromium coatings by electrolysis in a chromium electrolytic bath on a layer of electrolytically deposited metal as a base for the black chromium coating.

5. Method of producing black chromium coatings by electrolysis in a chromium electrolytic bath on a layer of electrolytically deposited metal belonging to the chromium-nickel-class, as a base for the black chromium coating.

ALFRED UNGELENK. JOHANNES FISCHER. mmz ENDRASS. 

